The syntrophins are a family of peripheral membrane adapter proteins that function in association with dystrophin, utrophin, and the dystrobrevins, all of which are proteins of the surface membrane of skeletal muscle and implicated in the important human disease, Duchenne muscular dystrophy (DMD). Alpha-syntrophin is the major syntrophin in muscle. In the alpha-syntrophin knockout mouse, the postsynaptic membrane at the neuromuscular junction shows major biochemical and morphological defects including low amounts of acetylcholine receptors and acetylcholinesterase, a complete absence of utrophin, immature appearing contacts, junctional folds that are disorganized and few in number, and altered distribution of AChR. Thus, there is a utrophin and NMJ support function of alpha-syntrophin. The first two aims of the project are to intended to extend current understanding that the other syntrophins of muscle (beta1, beta2, and probably gamma2) are not redundant with alpha-syntrophin. They are: 1) Test the hypothesis that transgenically expressed beta 1-syntrophin will restore utrophin to adult alpha-syntrophin 4- junctions but will not rescue other, or all other, aspects of the phenotype. 2) Test the hypothesis that transgenically expressed beta 2-syntrophin will restore no aspects of the alpha-syntrophin -/- phenotype. The results will provide a solid framework for molecular analysis of the mechanisms of the support function. Aim 3) Identify the critical functional domains of the alpha-syntrophin molecule by transgenic expression in alpha-syntrophin -/- mice of a) chimeric proteins containing domains of beta substituted into alpha-syntrophin (or the converse, alpha into beta) and/or b) alpha-syntrophin specifically mutagenized at selected sites. The results will identify domains and implicate syntrophin-dependent pathways. Aim 4) Determine whether low levels of utrophin mRNA, inability of the membrane to accept utrophin incorporation, or both contribute to the lack of utrophin at alpha-syntrophin -/- NMJs. If applicable, use these tools to analyze the transgenic mice. 5) Seek to identify determinants in the AChR accumulation pathway-- AChR mRNA levels, total AChR expression, cell surface AChR expression, AChR clustering response to agrin, stability of agrin-induced clusters--that may contribute to the low AChR content of alpha-syntrophin 4- NMJs. If applicable, use the understanding so generated to analyze the transgenic mice.